This invention relates to methods of precisely controlling stoppage and suck back of work liquids and to an improved pinch valve apparatus for practice of such method with both the method and apparatus being particularly useful for controlling a wide variety of work liquids including those with particulates therewithin. The method and apparatus features adjustable precision suck back on shutoff of flow.
Controlling the flow of liquids by pinching a resilient tube shut is well known. In its simplest form, the homeowner who bends the water hose pinches off the flow therein. Also, pinching off the flow by moving the resilient walls of a tube toward each other by application of a force at essentially right angle (transverse) to the tube is well developed practice. It is also well known that either one or more portions of the tube walls may be moved inwardly in the pinching operation, and various fixed ramps may be employed of both internal and external kinds to receive the movable tube wall to pinch off flow. In controlling flow of work liquids with particulates therewithin, such as liquid solder materials, epoxy resin components and the like, the repeated pinching (for example in excess of 100,000 cycles) at a single site has a tendency to abrade the resilient tube thereat, may cause cold flow or other distortions in the resilient tube at that site, and the pinching action under some circumstances may cause a partial separation of components in the work liquid being controlled. Also single site constriction of a resilient tube does not prevent drip of work liquid on shutoff at a downstream outlet with the resultant mess which is particularly undesirable in precision dispensing of work liquids used in assembly operations of various kinds. By creation of a partial vacuum intermediate the outlet and the shutoff site, a suck back of work liquid at the outlet and retention thereof in the system may be provided so as to eliminate drip on shutoff.
A partial vacuum may be created by simultaneously constricting 2 sites on a resilient tube and then removing the constricting force from the downstream site while maintaining the constriction of the upstream site. However this method does not overcome the aforenoted wear, cold flow and separation problems. Also, it does not afford easy adjustability of suck back for a variety of conditions and work liquid materials.
The improved method disclosed herein is elegantly simple, is easy to practice with a variety of pinching mechanisms, and overcomes the aforementioned prior art problems. The method comprises the steps of introducing the compression constriction force to a resilient tube to start constriction, moving the constricting force in an upstream direction while increasing the constriction force until shutoff is obtained and thereafter, while the shutoff is maintained, further moving the constriction force along the tube in an upstream direction to cause the tube at the original shutoff site (and each site along the travel path up to final stop position) to return toward its uncompressed state thereby creating a partial vacuum for suck back. The amount of travel from the original shutoff site to final stop directly affects the amount of partial vacuum created in the system, and by adjusting the amount of travel after initial shutoff to final stop, the amount of suck back can be easily adjusted for a variety of conditions and work liquid materials. It is preferred that the compression restriction creating force be applied with a rolling action and in a semi-resilient manner along the tube which reduces wear on the tube and better accomodates to particulates in the work liquid.
Various types of mechanical approaches to practicing prior art methods have been devised which are commonly known as pinch valves of two major types, the internal and external. The internal types of pinch valve necessarily restrict flow per unit size of diameter flexible tube and are not well adapted for controlling some work materials such as those that contain particulates since the supporting internal surfaces may cause undesired eddy currents in the flow, be subject to wear or separation of or collection of particulates. Also complete disassembly to adjust the cooperating parts is required. Some examples are shown in U.S. Pat. Nos. 3,840,207 and 3,830,462.
External pinch valves generally compress a resilient tube by transverse to the flow of work material movement of a one or a pair of movable members against a ramp or against each other at a single location. These pinch valves create no internal vacuum adjacent the shutoff site with consequent drip of work material from an outlet as discussed. Examples of this type of pinch valve are shown in U.S. Pat. Nos. 2,842,331; 3,759,483; 3,932,065 and 4,099,700. Because of the geometry, these prior art devices provide apparatus where the flexible tube at the shutoff site(s) is subject to wear due to repetitive concentration of and maintenance of all pinch force thereat. Also for applications requiring a cycling capability in excess of 250 M, this transverse movement of the shutoff pinching member or members is not suitable over time in that it can produce cold flow or other anomalies in the resilient member.
The novel pinch valve for practicing the method disclosed herein is extremely useful for applications for automatically actuated precision dispensing of hard to dispense liquid work materials such as epoxy resins, liquid solder pastes and the like in assembly operations where repeatable dependable startup and shutoff is required and adjustability to meet variability in work or work materials is necessary. It has been found dependable in operation in excess of 500 M cycles in the dispensing of the liquid resin components of epoxy resin systems. In assembly operations where parts are moved to a station adjacent an outlet from a liquid dispenser, it is particularly important that the outlet not drip liquid work material after shutoff and while a new work piece is being moved to position adjacent the outlet or during other shutoff times.
The apparatus features of movable car assembly having a roller movable into engagement with and gradually compressing a resilient tube against a ramp, the parts being arranged so as to cause the tube to be fully compressed at a site upstream from initial engagement. The roller is preferably semi-resilient. The car and the ramp surfaces are aligned in a manner to permit travel of the roller in an upstream direction after complete pinchoff of the flow to a final stop so that as the roller moves, the tube will return toward its normal uncompressed state creating a partial vacuum. The final stop position of the car is adjustable so as to adjust the amount of travel and hence the amount of suck back. The downstream side of the ramp is curved away from the straight path of the car roller so that easy to accomplish straight line movement of the car will still provide a gradual compression of the ramp overlaying tube along the length thereof. The car may be biased toward final stop position and actuated by a pneumatic or other actuator to its spaced from the tube position to provide a biased "closed" actuated " open" pinch valve. The exact initial position of the car is also adjustable. The valve also features easy access to replace the resilient tube after several hundred thousand cycles and an open bottom for quick drainage of the work liquid from the moving parts in the event of unexpected bursting of the tube.
The valve is well adapted for mass manufacturing techniques and is easily serviced by relatively unskilled personnel when in use in the field. The semi-resilient roller and trapped resilient tube being compressed on a fixed ramp accomodate well to hard to dispense liquids with particulates therein without the degree of wear of prior art devices. The valve is compact, easy to ship, versatile in its applications, may be easily assembled by others as a component to a larger system, can be made in a wide range of sizes, has few critical tolerances, all while providing dependable repeatable extremely long lived precision shutoff of flow with suck back.